1. Field of the Invention
The invention relates to an electrochemical cell which can be used for the electrolytic deposition of metals from dilute solutions when high depletion rates are required.
2. Description of the Prior Art
Known electrochemical cells with plate or mesh electrodes (c.f. C.L. Mantell; Electrochemical Engineering, McGraw Hill, Inc., New York, 1960) provide such low space-time yields when the concentrations of the metals to be deposited are less than 1 to 5 g/1, that they are not used industrially for the electrolysis of dilute solutions. In the above mentioned case, considerably better space-time yields can be attained with the likewise known porous electrodes or packed bed or fluidized bed electrodes (c.f. e.g. D. N. Bennion, J. Newman; J. Appl. Electrochem., Vol. 2 (1972), p. 113/A. K. P. Chu, M. Fleischmann, G. J. Hills; J. Appl. Electrochem. Vol. 4, (1974). p. 323/M Fleischmann, J. W. Oldfield, L. Tennakoon; J. Appl. Electrochem., Vol. 1, (1971), p. 103/H. R. Backhurst, M. Fleischmann, F. Goodridge, R. E. Plimley; British Patent Specification No. 1,194,181/F. J. Wilkinson, K. Haines; Trans. Inst. Mining Met. (Section C), Vol. 81 (1972), p. 157/D. S. Flett; Chem. and Ind. (1971), p. 300; G. S. James; U.S. Pat. No. 3,945,892); such electrodes will be referred to hereinafter as three-dimensional electrodes. Thus, for example, whereas the electrolysis of a solution containing 10 ppm of silver in a cell as used in the refining of silver, provides a space-time yield of 0.0017 g/1h, with the same solution a space-time yield of approximately 1.8 g/1h can be obtained in a packed bed cell.
That dimensional of a three-dimensional electrode that is parallel to the direction of flow of electric current is limited by what is known as the effective bed depth, since electrochemical converson takes place only within this depth. Conversely, the distance between feeder electrode and diaphragm which is given by design is called geometrical bed depth in the following description. Hitherto, this former limitation has been considered to be limitative for the applications since it has been assumed that the effective bed depth is generally only in the order of magnitude of 1 cm (cf. M. Fleischmann, J. W. Oldfield, J. Electroanal. Chem., Vol. 29, (1971), p. 211).
All the above-mentioned known electrochemical cells with three-dimensional electrodes have a constant or a linearly increasing geometrical bed depth. With a view to reducing outlay costs, i.e., of reducing the quantity of electrode and membrane materials to be used, it is desirable to oppose a given counter-electrode area or diaphragm area by the largest possible volume of three-dimensional electrode. However, this volume is limited by the fact that the space-time yield is reduced near the effective bed depth.
One object of the present invention is to provide a new electrochemical cell wherein the space-time yield is greater than that of the normal electrochemical cells.
Further objects are the realization of a very large concentration decrease per pass and a high current efficiency by a new electrochemical cell.
These and other objects are achieved by the present invention as described below and defined in the attached claims.